Cancer is the leading cause of death, second only to heart disease in both men and women. In the fight against cancer, numerous techniques have been developed and are the subject of current research to understanding the nature and cause of the disease, and to provide techniques for control or cure thereof.
Breast cancer is the most common tumor in women, representing 32% of all new cancer cases and causing 18% of cancer-related deaths of women in the United States. It is estimated that in 1995 a total of 183,400 new patients were diagnosed with breast cancer and 46,240 of these will die of this disease. In women, breast cancer is the second major cause of cancer deaths. Although the five-year survival rate for localized breast cancer has risen from 78% in the 1940's to 93% today, if the cancer has spread with distant metastases at the time of diagnosis, the five-year survival rate is only 18%. Thus, currently, a major challenge in the treatment of breast cancer is to provide a cure for patients who have advanced metastatic disease. Although metastatic breast cancer is sensitive to chemotherapy, it remains incurable with contemporary therapeutic approaches. While a majority of patients experience an initial response, their overall survival is only modestly improved. The development of new and potent anti-breast cancer drugs and the design of treatment protocols utilizing these new agents is an exceptional focal point for research in the modern therapy of breast cancer.
Drug targeting is a potentially attractive new approach to killing malignant cells, while leaving normal tissue unharmed. The advent of hybridoma technology made the supply of monoclonal antibodies limitless. Monoclonal antibodies linked to bioactive agents form immunoconjugates, combining selectivity of the carrier moiety with potency of the bioactive moiety.
For the past decade, immunoconjugates have been under investigation for the treatment of various cancers. Although these agents have shown some potential to provide safe and effective therapy for human disease, many difficulties remain. Ideally, consistently locateable and reliable markers on target cells would permit the binding portion of an immunoconjugate to completely avoid non-target tissues. In reality, however, cross reactivity with antigens expressed by vital life-maintaining organs can give rise to unacceptable complications. Patients may also develop immune responses to the separate components of the immunoconjugate. Moreover, cytotoxicity solid tumor penetration, and relapse caused by residual disease present treatment problems. Therefore, there is a continuing need for improved agents and methods of their use in targeting and inhibiting or eliminating cell populations associated with metastatic disease.
The human epidermal growth factor (EGF) is a six kilodalton (kDa), 53 amino acid, single-chain polypeptide which exerts its biological effect by binding to a specific 170 kDa cell membrane receptor (EGF-Rc). The human EGF-Rc consists of an extracellular domain with a high cysteine content and N-linked glycosylation, a single transmembrane domain, and a cytoplasmic domain with tyrosine kinase activity.
Many types of cancer display enhanced EGF-Rc expression on their cell surface membranes. Enhanced expression of the EGF-Rc can increase signalling via receptor-mediator pathways which lead to pleiotropic biological effects including excessive proliferation and metastases. Examples include prostate cancer, breast cancer, lung cancer, head and neck cancer, bladder cancer, melanoma, and brain tumors. In breast cancer, expression of the EGF-Rc is a significant and independent indicator for recurrence and poor relapse-free survival. The epidermal growth factor receptor (EGF-Rc) on cancer cells therefore represents a potential target for biotherapy.
Activation of the proliferative pathways by protein tyrosine kinases has been suggested to play a role in the development and progression of various types of human cancer. For example, SRC kinase has been suggested to play a role in the pathogenesis of breast cancer, the enzymatic activity of SRC in breast cancer being significantly higher when compared to benign or normal breast tissues.
Genistein, an isoflavone (5, 7, 4'-trihydroxyisoflavone) from fermentation broth of Pseudomonas, is a naturally occurring tyrosine kinase inhibitor present in soy beans. Genistein, at very high concentrations, which is not achievable in vivo, has been shown to inhibit the in vitro proliferation of cancer cells, including human breast cancer cells (Monti et al., 1994, AntiCancer Res., 14:1221-1226) and prostate cancer cells (Peterson et al., 1993, Prostate 22:335-345).
Genistein is generally known as a weak tyrosine kinase inhibitor which only reversibly inhibits tyrosine kinase (Levitzki and Gazit, Science 267:1782-1788, 1995). Furthermore, genistein has been demonstrated to prevent "death" signals triggered by enhanced tyrosine kinase activity, including radiation-induced cell death, (Uckun et al., PNAS, USA 89:9005, 1992).
Genistein and similar isoflavones do not easily enter cells, and its delivery to cancer cells in non-toxic doses presents problems for therapeutic use. While genistein and similar tyrosine kinase inhibitors function well against cancer cells in vitro, such effects are observed at very high concentrations not achievable in vivo without excessive toxicity. Therefore, despite potent inhibition of TK activity in vitro, many TK inhibitors, including genistein, have shown little or no TK inhibitory activity, in vivo (Levitzki and Gazit, 1995, Science 267:1782-1788). Genistein, a pure competitive inhibitor of ATP, has been demonstrated as inhibitory only in the 10-100 .mu.M range in tissue culture, and showed no efficacy in vivo.
Although the prior art suggests various types of therapeutic models for the treatment of metastatic disease, including breast cancer, to date useful agents have not been adequately demonstrated. There remains a need for specific and effective therapeutic agents for the treatment of metastatic disease.